Aircraft surfaces are typically made of a metal, such as aluminum or titanium. A primer can be coated on the metal surface to prevent or reduce corrosion. However, because primers do not adequately adhere to the metal surfaces, adhesive coatings are typically disposed between a metal surface and a primer to promote adhesion between the metal and the primer.
An adhesive sol-gel film may be disposed at the interface between the metal and primer. After extended use of the aircraft surface, pores may form within a sol-gel film. The pores retain water over time, which promotes corrosion of the metal surface. Typical sol-gel films do not inherently possess corrosion resistance properties. Furthermore, the presence of water within the pores is subject to an increase of osmotic pressure within the pore which creates a visually noticeable defect in the aircraft surface known as a “blister”.
Corrosion protection of aircraft metal surfaces has typically relied on primers having hexavalent chromium. However, hexavalent chromium is a carcinogen and is toxic to the environment. Thus, there is regulatory pressure to eliminate the use of hexavalent chromium from primers and pretreatments. Furthermore, corrosion inhibitors have been added to sol-gel films (or included in the formation of the sol-gel film), but these inhibitors have been found to decrease both the adhesive ability of the sol-gel film and anticorrosion ability of the corrosion inhibitor when present in the sol-gel film.
Therefore, there is a need in the art for new and improved corrosion resistant, adhesive sol-gel films.